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gc.man
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gc.man
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.TH BDWGC 3 "23 Aug 2023"
.SH NAME
GC_malloc, GC_malloc_atomic, GC_free, GC_realloc, GC_enable_incremental,
GC_register_finalizer, GC_malloc_ignore_off_page,
GC_malloc_atomic_ignore_off_page, GC_set_warn_proc \- Garbage collecting
malloc replacement
.SH SYNOPSIS
#include <gc/gc.h>
.br
void * GC_malloc(size_t size);
.br
void * GC_malloc_atomic(size_t size);
.br
void GC_free(void *ptr);
.br
void * GC_realloc(void *ptr, size_t size);
.br
void GC_enable_incremental(void);
.br
void * GC_malloc_ignore_off_page(size_t size);
.br
void * GC_malloc_atomic_ignore_off_page(size_t size);
.br
void GC_set_warn_proc(void (*proc)(char *, GC_word));
.br
.sp
cc ... -lgc
.LP
.SH DESCRIPTION
.I GC_malloc
and
.I GC_free
are plug-in replacements for standard malloc and free. However,
.I
GC_malloc
will attempt to reclaim inaccessible space automatically by invoking
a conservative garbage collector at appropriate points. The collector
traverses all data structures accessible by following pointers from the
machines registers, stack(s), data, and bss segments. Inaccessible structures
will be reclaimed. A machine word is considered to be a valid pointer if
it is an address inside an object allocated by
.I
GC_malloc
or friends.
.LP
In most cases it is preferable to call the macros GC_MALLOC, GC_FREE, etc.
instead of calling GC_malloc and friends directly. This allows debugging
versions of the routines to be substituted by defining GC_DEBUG before
including gc.h.
.LP
See the documentation in the include files gc_cpp.h and gc_allocator.h,
as well as the gcinterface.md file in the distribution,
for an alternate, C++ specific interface to the garbage collector.
Note that C++ programs generally
need to be careful to ensure that all allocated memory (whether via new,
malloc, or STL allocators) that may point to garbage collected memory
is either itself garbage collected, or at least traced by the collector.
.LP
Unlike the standard implementations of malloc,
.I
GC_malloc
clears the newly allocated storage.
.I
GC_malloc_atomic
does not. Furthermore, it informs the collector that the resulting object
will never contain any pointers, and should therefore not be scanned by the
collector.
.LP
.I
GC_free
can be used to deallocate objects, but its use is optional, and generally
discouraged.
.I
GC_realloc
has the standard realloc semantics. It preserves pointer-free-ness.
.I
GC_register_finalizer
allows for registration of functions that are invoked when an object becomes
inaccessible.
.LP
The garbage collector tries to avoid allocating memory at locations that
already appear to be referenced before allocation. (Such apparent
``pointers'' are usually large integers and the like that just happen to look
like an address.) This may make it hard to allocate very large objects.
An attempt to do so may generate a warning.
.LP
.I
GC_malloc_ignore_off_page
and
.I
GC_malloc_atomic_ignore_off_page
inform the collector that the client code will always maintain a pointer to
near the beginning (i.e. within the first heap block) of the object, and that
pointers beyond that can be ignored by the collector. This makes it much
easier for the collector to place large objects. These are recommended for
large object allocation. (Objects expected to be > ~100 KB should be
allocated this way.)
.LP
It is also possible to use the collector to find storage leaks in programs
destined to be run with standard malloc/free. The collector can be compiled
for thread-safe operation. Unlike standard malloc, it is safe to call malloc
after a previous malloc call was interrupted by a signal, provided the
original malloc call is not resumed.
.LP
The collector may, on rare occasion, produce warning messages. On UNIX
machines these appear on stderr. Warning messages can be filtered,
redirected, or ignored with
.I
GC_set_warn_proc
This is recommended for production code. See gc.h for details.
.LP
Fully portable code should call
.I
GC_INIT
from the primordial thread of the main program before making any other
GC calls. On most platforms this does nothing and the collector is
initialized on first use. On a few platforms explicit initialization is
necessary. And it can never hurt.
.LP
Debugging versions of many of the above routines are provided as macros.
Their names are identical to the above, but consist of all capital letters.
If GC_DEBUG is defined before gc.h is included, these routines do additional
checking, and allow the leak detecting version of the collector to produce
slightly more useful output. Without GC_DEBUG defined, they behave exactly
like the lower-case versions.
.LP
On some machines, collection will be performed incrementally after a call to
.I
GC_enable_incremental.
This may temporarily write protect pages in the heap. See the README file for
more information on how this interacts with system calls that write to the
heap.
.LP
Other facilities not discussed here include limited facilities to support
incremental collection on machines without appropriate VM support, provisions
for providing more explicit object layout information to the garbage
collector, more direct support for ``weak'' pointers, support for
``abortable'' garbage collections during idle time, etc.
.LP
.SH "SEE ALSO"
The README and gc.h files in the distribution. More detailed definitions of
the functions exported by the collector are given there. (The above list is
not complete.)
.LP
The web site at http://www.hboehm.info/gc/ (or https://github.com/ivmai/bdwgc/).
.LP
Boehm, H., and M. Weiser, "Garbage Collection in an Uncooperative Environment",
"Software Practice & Experience", September 1988, pp. 807-820.
.LP
The malloc(3) man page.
.LP
.SH AUTHOR
Hans-J. Boehm (boehm@acm.org).
Some of the code was written by others (see the AUTHORS file for the details),
most notably by Alan Demers, and, recently, Ivan Maidanski.